During an acute asthma attack, there is a marked exacerbation of the underlying inflammation in the small and medium sized airways in the lungs. The lining of the airways becomes edematous, and the surrounding smooth muscle contracts. The transmission of air becomes restricted and turbulent resulting in ventilation/perfusion (V/Q) mismatching and hypoxemia.
Acute asthma attacks are often initially treated with an inhaled beta adrenergic agonist that simulates the .beta.-receptors on airway smooth muscles causing bronchodilatation. However, .beta.-agonists do not treat the underlying inflammation and, as the asthma attack develops, the .beta.-receptors tend to become refractory to .beta.-agonist medication. Acute asthma also causes hypoxemia which is conventionally treated with oxygen. Hypoxemia is often associated with increased bronchospasm, bronchia reactivity and anxiety, all of which can be ameliorated with oxygen therapy.
The initial effect of .beta.-agonist medication in an acute asthma attack is to cause the pulmonary vasculature to dilate which tends to exacerbate the edema. This results in a further deterioration in the V/Q mismatching and hypoxemia. As a general rule, a .beta.-agonist medication should always be given with oxygen in an acute asthma attack. Currently, oxygen is administered under medical supervision. It would obviously be beneficial for patients, for example asthmatics, to be able to self-administer oxygen at the onset of an attack so as to optimize their treatment. However, for patients with chronic obstructive airway disease, high partial pressure of oxygen can compromise their hypoxic drive.
It is known to administer a helium-oxygen gas mixture in obstructive airways disease. The usefulness of helium in such conditions is related to its physical properties. It is an inert gas without any pharmacological activity. It has a lower density than nitrogen or oxygen so that, when is mixed with oxygen, there results a lower airways resistance than with either oxygen alone or an oxygen-nitrogen mixture. The Reynold's number is reduced such that areas of turbulent flow in the inflamed airways are converted to laminar flow.
In the distal airways, the low solubility of helium prevents any significant absorption into the pulmonary vasculature. This prevents atelectasis formation that can occur with 100% oxygen (peripheral lung collapse). There is laminar flow in the distal airways that is dependent on the viscosity of the inhaled gas. Although helium is relatively viscous compared to oxygen or nitrogen, there is still a net gain in overall gas flow in acute asthma using a helium oxygen mix.
A helium:oxygen mixture of 80:20 volume percent has been shown to reduce pulses paradoxus and increase peak expiratory flow in patients with acute asthma. This reduces muscle fatigue, maintains arterial oxygenation and keeps the patient in relatively good condition until other forms of medication have had the opportunity to exert their effect. An additional benefit of a helium oxygen mixture is that when administered to patients with an acute myocardial infarction the myocardium appears to be stabilized reducing the risk of ventricular arrhythmias.
Another important benefit of oxygen is that, in MRI detection of cancer, the presence of oxygenated blood within the tumor can commonly improve the MRI imaging. This can be achieved in particular by having the patient inhale an enhanced level of oxygen. In addition, it has been found that the presence of carbon dioxide in the inhaled gas acts as a vasodilator in those blood vessels in and around the tumor thereby facilitating an increased level of oxygen in such vessels which acts to improve imaging. Typically, five volume percent of carbon dioxide (CO.sub.2) is added to the gas, although somewhat higher or lower concentration may be used to good effect in different circumstances.
While carbon dioxide can be administered in air or oxygen-enriched air, it is more commonly administered in admixture with pure oxygen. Such 5/95 vol. percent mixtures carbon dioxide and oxygen are commercially available. However, a problem inherent in such preparations is the pronounced physiological effects of a 5% C0.sub.2 concentration, including inability to breath normally, high ventilation rates, panic and production of heat which may lead to the patient being unable or unwilling to continue with the imaging or treatment. This is particularly troublesome for patients with obstructive pulmonary disease.
It has been recently proposed to reduce the carbon dioxide concentration in such preparations to 4 vol%, possibly 3 vol% and perhaps below in order to minimize such detrimental physiological effects without substantial loss of the benefits in the imaging process. The reduction in such effects must, however, be balanced against the need to obtain the best possible imaging.
Therefore, there is a need for a gas mixture which can be made readily available to all patients for self administration, for example at the onset of an acute asthma attack, or for use in imaging, for example magnetic resonance imaging (MRI) or for general medical use, and which can overcome the above difficulties.